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Wednesday, May 14, 2014

Understanding Fault Tree Analysis


Understanding Statistical Tools and Techniques

Write-up – 11

Understanding Fault Tree Analysis


Fault Tree Analysis (FTA) is a top down, deductive failure analysis method. In this analysis an undesired state of a system is analyzed using Boolean logic to combine a series of lower-level events. The fault tree analysis (FTA) was first introduced by Bell Telephone Laboratories. Bell Telephone Laboratories developed the concept of fault tree analysis in 1962 for the US Air Force for use with the Minuteman system. It is one of the most widely used methods in system reliability, maintainability and safety analysis. It is a deductive procedure used to determine the various combinations of hardware and software failures and human errors that could cause undesired events (referred to as top events) at the system level .This analysis method is mainly used in the fields of safety engineering and reliability engineering to understand how systems can fail, to identify the best ways to reduce risk or to determine (or get a feeling for) event rates of a safety accident or a particular system level (functional) failure. This analysis method is also used in the aerospace, nuclear power, chemical and process, pharmaceutical, petrochemical and other high-hazard industries; but is also used in fields as diverse as risk factor identification relating to service system failure. In aerospace, the more general term "system Failure Condition" is used for the "undesired state" / Top event of the fault tree. These conditions are classified by the severity of their effects. The most severe conditions require the most extensive fault tree analysis. These "system Failure Conditions" and their classification are often previously determined in the functional Hazard analysis.

FTA method can be used to understand the logic leading to the top event / undesired state. It can show compliance with the (input) system safety / reliability requirements. This method can prioritize the contributors leading to the top event by creating the Critical Equipment/Parts/Events lists for different importance measures. FTA method can monitor and control the safety performance of the complex system (e.g., is a particular aircraft safe to fly when fuel valve x malfunctions? For how long is it allowed to fly with the valve malfunction?). This method is helpful to minimize and optimize resources. This analysis method can assist in designing a system. The FTA can be used as a design tool that helps to create (output / lower level) requirements. This method can function as a diagnostic tool to identify and correct causes of the top event. It can help with the creation of diagnostic manuals / processes. The main purpose of the fault tree analysis is to help identify potential causes of system failures before the failures actually occur. It can also be used to evaluate the probability of the top event using analytical or statistical methods. These calculations involve system quantitative reliability and maintainability information, such as failure probability, failure rate and repair rate. After completing an FTA, you can focus your efforts on improving system safety and reliability.

FTA method is a deductive analysis, which begins with a general conclusion, then attempts to determine the specific causes of the conclusion by constructing a logic diagram called a fault tree. This method is also known as taking a top-down approach.

FTA Logic Diagram

The basic symbols used in an FTA logic diagram are called logic gates and are similar to the symbols used by electronic circuit designers. Fault tree diagrams consist of gates and events connected with lines. The AND and OR gates are the two most commonly used gates in a fault tree. To illustrate the use of these gates, you may consider two events (called "input events") that can lead to another event (called the "output event"). If the occurrence of either input event causes the output event to occur, then these input events are connected using an OR gate. Alternatively, if both input events must occur in order for the output event to occur, then they are connected by an AND gate. The following figure shows a simple FTA diagram in which either A or B must occur in order for the output event to occur. In this diagram, the two events are connected to an OR gate. If the output event is system failure and the two input events are component failures, then this fault tree indicates that the failure of A or B causes the system to fail.


When you perform an FTA, you systematically determine what happens to the system when the status of a part or another factor changes. In some applications, the minimum criterion for success is that no single failure can cause injury or an undetected loss of control over the process. In others, where extreme hazards exist or when high value product is being processed, the criteria may be increased to require toleration of multiple failures.

Fault Tree Construction


You can construct Fault Tree diagram by adhering to simple steps. You should first define the fault condition and write down the top level failure. By using technical information and professional judgements, you should determine the possible reasons for the failure to occur. You should continue to break down each element with additional gates to lower level. You should consider the relationships between the elements so that you can decide whether to use an AND or an OR gate. The two most commonly used gates in a fault tree are the AND and OR gates. You should consider two events (or blocks) comprising a Top Event (or a system). If occurrence of either event causes the top event to occur, then these events (blocks) are connected using an OR gate. Alternatively, if both events need to occur to cause the top event to occur, they are connected by an AND gate. You may finalize and review Fault Tree Construction. The chain generally terminated in a basic fault that may be human, hardware or software. If possible, then you should evaluate the probability of occurrence for each of the lower level element and calculate the statistical probabilities from the bottom up.

Bibliography

* What Is a Fault Tree Analysis? Use a general conclusion to determine specific causes of a system failure, Simha Pilot, Quality Progress, March 2002
* http://en.wikipedia.org/wiki/Fault_tree_analysis
* http://reliawiki.com/index.php/Fault_Tree_Diagrams_and_System_Analysis
* http://www.weibull.com/basics/fault-tree/